1. Field of the Invention
The present invention relates to a method for controlling the temperature of the printing head of an impact printer.
2. Description of the Prior Art
An impact printer, such as a serial printer, which is employed as an output unit of an electronic computer or the like is designed generally to print with a printing head mounted on a carriage while the carriage is moved along the platen.
A printing head provided with a printing stylus unit is used often for such a printer to meet the necessity of printing Chinese letters, patterns and graphs.
Such a printing head is mounted on a carriage which is reciprocated in predetermined directions by a driving motor and the printing styluses are actuated continuously by supplying electric current to the corresponding electromagnetic coils for dot printing operation.
Heat is generated by the electromagnetic coils as the electromagnetic coils are energized and this heat tends to heat the printing head. As the electromagnetic coils are energized successively and frequently, the heat generated by the electromagnetic coils is accumulated at a rate exceeding the rate of heat radiation from the printing head, and thereby the printing head is heated up to an excessively high temperature. Consequently, the electrical insulation of the electromagnetic coils is deteriorated, short-circuit accident is likely to occur and the parts disposed around the electromagnetic coils tend to be burnt or subjected to thermal deterioration.
Such overheating of the printing head occurs when the printing head is operated at a high printing duty rate, such as in continuously printing English letters or katakana letters (Japanese syllabic letters) or in printing pictures or graphic representations.
A method to obviate the overheat of the printing head has previously been proposed, in which method the printing head is provided with a thermal sensor for detecting the temperature of the printing head and when abnormal rise in the temperature of the printing head is detected by the thermal sensor, the operation of the printing head is interrupted to allow the printing head to cool off. However, this method has a disadvantage that the frequent interruption of the printing operation of the printing head during continuous printing operation reduces the printing capacity of the printer.
Another method has been proposed to eliminate such a disadvantage of the prior method. According to this method, reduction in the printing capacity of the printer resulting from the frequent interruption of printing operation is limited to a small extent and the burning of the electromagnetic coils of the printing head is prevented by driving the printing head for printing operation only while the printing head is moved in one direction and interrupting the printing operation of the printing head while the printing head is moved in the opposite direction after the temperature of the printing head has reached an upper limit of temperature.
However, when the printing head is operated at a high printing duty rate (rate of printing cycles per unit time), excessive rise in the temperature of the printing head is possible even if the printing operation is performed only while the printing head is moved in one direction, and hence there is the possibility of the electromagnetic coils being burnt out.